The present invention relates to a method and apparatus for transmitting process data in an automatically controlled installation and a corresponding process data receiver designed to receive process data according to the method.
WO 2004/097539 A1 describes a method and a device for controlling a safety-critical process in which a failsafe operation is required in order to avoid a risk to persons. In particular, it involves the monitoring and control of installations operating in an automated manner, the operation of which presents a risk of injury to persons. An installation of this type is usually safeguarded by protective fencing, protective doors, light barriers, light grids, emergency switches and other safety sensors. As soon as a protective door is opened, a light barrier is interrupted or an emergency switch is actuated, the installation must be stopped or transferred into a safe condition in a different manner. This is normally achieved by a controller of failsafe design evaluating state signals of the protective doors, light grids, emergency switches, etc., at defined recurring time intervals and by triggering safety-related actuators depending thereon, such as, for example, contactors, which are disposed in the power supply path to the electric drives of the installation.
The sensors and actuators are often physically distanced from the controller. In a large-scale installation with many sensors and actuators, it is appropriate to connect the controller to the sensors and actuators via a bus system. The bus system enables the exchange of digital data which represent the states of the sensors and actuators, and also control commands of the controller. These data are referred to below generally as process data. In addition, process data can contain diagnostic information and/or configuration information which is important for the operation of the installation.
Aforementioned WO 2004/097539 A1 describes a method and a corresponding device, wherein a plurality of I/O units transmit process data via a bus system to a control unit. The control unit generates a variable keyword with which the I/O units encode the process data to be transmitted. Due to the changing coding, the transmitted data change, even if the process data as such remain the same over a long period of time, e.g. because the state of a protective door does not change. A defined dynamic behavior is created using the variable keyword, enabling the controller to check the I/O units for failsafe operation. The method and the corresponding device have proven successful in practical operation. Nevertheless, the wish exists to further optimize the transmission of process data in an installation controlled in an automated manner. In particular, the wish exists to increase the data transmission speed in order to transmit as much process data as possible in the shortest possible time intervals via a bus system that is simple and economical to implement.
WO 2006/069691A1 describes a further control system with physically distributed stations between which process data are transmitted at recurring time intervals. A first station produces a data frame with a plurality of data fields. The data frame is transmitted in the manner of a bucket brigade by a series of further stations. Each further station writes process data into the data frame and reads process data from other stations when the data frame filled by all stations is transmitted back again by the series of stations to the first station. This method makes it possible to implement a cross-communication between any stations of a control system within a single communication cycle.
The two aforementioned documents describe methods for transmitting process data in automatically controlled installations in a higher protocol layer of the OSI reference model. To put it more precisely, this involves rules according to which the communication proceeds. The electrical or optical signals with which the process data are transmitted is of secondary importance for the proposed methods. In particular, it is irrelevant for the proposed methods whether the process data are transmitted using separate data and clock lines or whether the process data and the transmission clock are combined in one signal. The clock signal determines the transmission speed and a combination in one signal is desirable in order to minimize the number of transmission lines.
Methods already exist in the prior art which make it possible to combine a clock signal and data in one common signal. These methods typically require a PLL on the data receiver side. The PLL (Phase Locked Loop) is a control circuit which generates a clock signal which synchronizes itself with an externally fed signal. A clock signal can thus be generated from a data stream using a PLL, said clock signal then enabling an unambiguous reconstruction of the individual data symbols of the data stream. Unfortunately, a PLL is fairly complex and costly if a large number of relatively simple and economical process data receivers are to be implemented.
Alternatively, the possibility exists for reconstructing data symbols from a serial data stream using oversampling. In this case, the data receiver must process the received data stream at a clock frequency that is substantially higher than the symbol rate of the data stream itself. This is problematic if the symbol rate of the data stream and accordingly the data transmission speed are intended to be high, since very fast and correspondingly costly circuits are then required on the receiver side. The oversampling is therefore typically used only for relatively slow data transmissions, such as, for example, in the known RS-232 data transmission from the 1960s. An oversampling in an RS-232 data transmission is described, for example, in an Application Note with the number 2141 from Maxim Integrated Products, based in Sunnyvale, Calif., USA.